You could start by looking at stock price. TSLA is flat since 2020, and their only release in a long time is the Cybertruck which is frankly terrible. They've also lost a huge amount of marketshare. Oh, and Musk's outspoken political opinions are toxic to most EV buyers and have massively turned off a large part of the fanbase that originally propelled him to success.

Twitter is a complete dumpster fire (revenue down 80% since he acquired it, and is now making only around 10% of the money required to even service the debt from acquisition). Basically every company he's involved with is doing terribly, except for SpaceX, which fortunately has Gwynne Shotwell at the helm. But even SpaceX isn't doing so well with their latest rocket, Starship; the upper stage is simply way too big and heavy for most uses, and will require up to 15 (!!) additional refueling in-orbit refueling launches to fully refuel a single Starship for missions beyond LEO, like the upcoming lunar missions. This is a huge liability that casts doubt on the overall architecture. Contrast with the Saturn V, a smaller rocket that could nevertheless do an entire self-contained lunar mission with a single launch.

I was a huge Elon fan for many years, but everything he's been doing recently has been a swing and a miss.